1. Field of the Invention
This application relates in general to a pressure sensor and in particular to a pressure sensor having a ballast resistive element.
2. Description of the Related Art
Referring to FIG. 1, a conventional pressure sensor comprises two substrates 10 and 109 disposed on the top and bottom sides therefore. Two metal electrodes 11 and 119 are disposed on the substrates 10 and 109, and two piezoresistive layers 12 and 129 are disposed on the metal electrodes 11 and 119 with a space 16 formed therebetween. Additionally, a spacer 15 is disposed between the substrates 10 and 109, wherein a part of the spacer 15 is extended between the piezoresistive layers 12 and 129 to form the space 16. As shown in FIG. 1, the metal electrodes 11 and 119 are electrically connected to a circuit system C. When no pressure is applied to the pressure sensor, the sensing circuit is open. When a pressure P is applied to the pressure sensor, as shown in FIG. 2, the piezoresistive layers 12 and 129 contact each other and form a closed circuit, thus enabling pressure measurement.
Since the piezoresistive layers 12 and 129 are made of piezoresistive material, they can have small resistance when deformed by external pressures. In the conventional pressure sensor, output resistance of the pressure sensor decreases with the increase of the pressure P.
According to Ohm's law (V=IR), the output resistance of the pressure sensor dominates the output current. Hence, the current I will increase when the voltage V is fixed with the decrease of the pressure P. However, when an overload pressure is applied, the linear pressure sensor may have a very small resistance that results in excessive output current. Thus, the circuit system can be damaged by the current.
As depicted in FIG. 2, when the pressure P is exerted on the pressure sensor, the substrate 10, the metal electrode 11, and the piezoresistive layer 12 are deformed downwardly, wherein the piezoresistive layers 12 and 129 contact each other. Output resistance of the piezoresistive layers 12 and 129 is determined by the height h thereof.
Resistance output of a piezoresistive sensor can be calculated by the formula R=p*L/A, wherein R is the electrical resistance output of the piezo resistive sensor (measured in ohms, Ω), L is the total thickness of the pressed piezoresistive layers (measured in centimeters, cm), and A is the pressed area applied over the piezoresistive sensor (measured in square centimeters, cm2).